This invention relates to image scanners for providing document and picture information to computers, in particular to flat bad image scanners.
A flat bed image scanner is widely used today is a periphery equipment for scanning documents, books and pictures and for furnishing the scanned information to a computer to be processed. However, traditional flat bed image scanner leaves much to be desired in terms of cost and volume. In other words, it is desirable to develop a smaller and less costly flat bed image scanner.
FIG. 1 shows the structure of a conventional flat bad image scanner, which includes an image reading sensor 11c, a glass window 5, a sliding guide 7, a frame 12, driving mechanism (not shown) and image processing circuit (not shown). The principle of operation is as follows:
A document or picture 13 is placed over a window 5. The image 6 is irradiated with a light source 11a which is mounted on an optical carriage 11. The reflected light L passes through a group of reflecting mirrors 111, 112, 113 and lens 11b, and impinges on an image sensor 11c. Signals sensed by the sensor 11c are processed by electronic circuits (not shown) to be digitized. After the sensor 11c completes scanning the signals from one row or one area, a translating mechanism (not shown) moves the carriage 11 along the guide 7 to a next scanning position. The carriage 11 is held in place by a flange 11d which wraps around the guide 7. This scanning continues until complete image information is read.
The biggest drawback of such a carriage 11 containing the light source 11a, the reflecting mirrors 111, 112, 113, the sensors 11c and the flange 11d, lies in its bulkiness and the long light path between the image appearing at position 6 and the reflected image appearing at the sensor 11c. Take a standard size A4 paper, the distance between position 6 and the sensor 11c is more than 26 cm. In a conventional xe2x80x9cflat bed image scannerxe2x80x9d with three reflecting mirrors, the optical carriage has a cross-sectional area of approximately 9 cmxc3x976 cm.
It is desirable to reduce the size, the power consumption and the cost of a flat bed image scanner. One approach to overcome the problem is to forego the mirror and lens system as the basic element and use a xe2x80x9cContact Image Sensorxe2x80x9d CIS to construct a flat bed image scanner. The flat bed image scanner can achieve small size. The contact image sensor used in the present invention as shown in FIG. 2 measures only 2 cmxc3x972 cm approximately, containing a light source 1a, a focusing lens 1b, an image sensor 1c, a dust shielding window 1d, and a frame 1e. Position 6a is where an object is scanned. However, the contact image sensor has a drawback in that the depth of focus is very short, only 0.3 mm in depth. In other words, if position 6a is off by 0.3 mm upwards or downwards, the image becomes fuzzy. On the other hand, the traditional optical system 11 shown in FIG. 1 has a depth of focus as deep as 5 mm.
Thus, it is necessary to overcome the problem of shallow depth of focus for a flat bed image scanner. The traditional optical system shown in FIG. 1 cannot be used directly for a contact image sensor, because when a contact image sensor rides on a track to travel, it is difficult to maintain an error of less than 0.3 mm between the object and the contact image sensor. Any fluctuation in mechanical dimension or change in temperature can cause deformation in material and renders mass production difficult.
An object of this invention is to devise a structure adapted to the shallow depth of focus of a contact image sensor for use in a flat bed image scanner. Another object of this invention is to reduce the optical path in a flat bed image scanner. Still another object of this invention is to reduce the cross-sectional area of a flat bed image scanner.
These objects are achieved in this invention by using a novel structure for the contact image sensor. The contact image sensor is pressed against the glass window by a spring, so that the sensor can slide in intimate contact with the glass window. Thus, the distance between the scanned object and the contact image sensor can be held within an error 0.3 mm. Any error in mechanical precision or due to temperature fluctuation cannot adversely affect the scanning quality of the image scanner.